System and method for brownout protection of a FET based battery switch

ABSTRACT

A switching circuit operative to switch between a first and second power source to an output is disclosed. The circuit comprises a MOSFET switch electrically connected to the first and second power sources and operative to provide power from either one of the power sources to the output. The circuit includes a RC network electrically connected to a gate of the MOSFET switch. The RC network is operative to delay switching between the first and second power sources such that the second power source has the opportunity to fully ramp-up in order to provide continuous power to the output. Accordingly, the second power source can fully provide power to the output when the power is switched from the first power source to the second power source.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.10/279,603, entitled “System and Method for Brownout Protection of a FETBased Battery Switch,” filed on Oct. 23, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to switching between two powersources and more particularly to a system and method of switching powerbetween an internal battery and an external DC adapter without loss orinterruption of power.

2. Status of the Prior Art

Electronic devices such as laptop computers, cell phones, PDA's, etc. .. , can be powered using either an internal battery power source or anexternal power source. For example, a cell phone may use a Li-Ionrechargeable battery as the internal power source and a DC adapter asthe external power source. When the external power source is pluggedinto the device, it disables power from the internal power source sothat the external power source is the sole provider of power. Similarly,when the external power source is removed from the device, power isswitched to the internal power source.

The switching between the internal power source and the external powersource can be accomplished through the use of a FET switch. Referring toFIG. 1, a prior art switching circuit 10 using a MOSFET switch U4 isshown. The MOSFET switch U4 has switching transistors 12 and 14, as wellas diodes 16 and 18. The prior art switching circuit further includesresistor R102 and diode D10, as shown in FIG. 1.

When the DC adapter is plugged into the device, the voltage V_(ADAPTER)increases gradually and the MOSFET switch U4 is turned off therebyswitching off the power from V_(BAT). However, there is a delay insupplying the power from the DC adapter such that the voltage at V_(OUT)will drop to zero. A switching voltage regulator that draws current allthe time may be connected to V_(OUT). The delay in supplying power fromthe DC adapter creates the situation whereby there may be no currentgoing to the switching regulator thereby resulting in 0 volts at V_(OUT)for a short period of time. This voltage drop on the switching regulatoris transferred to the output of the regulator such that brownoutconditions may be created on sensitive digital integrated circuits.

Similarly, when the DC adapter is unplugged from the device, the MOSFETswitch U4 will switch power back to the battery. Yet, when the device isunplugged, the voltage on the gates of the switching transistors 12 and14 will decrease slowly. During this period of discharge, the MOSFETswitch U4 is not fully on and incapable of supplying enough current fromthe battery. As such, there is again a drop in voltage at V_(OUT).

The present invention addresses the above-described deficiencies inswitching between power supplies by providing a circuit whichinstantaneously switches between power sources. The switching circuit ofthe present invention avoids extensive voltage drop at the output of theswitching circuit during DC adapter turn-on and turn-off such thatbrownouts are avoided.

SUMMARY OF THE INVENTION

In accordance with a preferred embodiment of the present invention,there is provided a switching circuit operative to switch between afirst and second power source. The first power source may be an internalbattery to a device whereas the second power source may be an externalDC adapter. The circuit comprises a MOSFET switch electrically connectedto the first and second power sources and operative to provide powerfrom either one of the power sources to the output. The circuit furtherincludes a RC network electrically connected to the gate of the MOSFETswitch. The RC network is operative to delay switching between the firstand second power sources such that the second power source has theopportunity to ramp-up in order to provide continuous power to theoutput. Accordingly, the second power source can fully provide power tothe output when the power is switched from the first power source to thesecond power source.

The RC network has a time constant that is greater than the switchingconstant of the MOSFET switch. This allows power from the second powersource to fully ramp-up before being fully switched by the MOSFETswitch. Typically, the time constant of the RC network is five timegreater than the switching constant of the MOSFET switch.

The circuit further includes a diode electrically connected between thefirst power source and the output. The diode can provide power from thefirst power source to the output when the power is switched from thesecond power source to the first power source. Accordingly, power can becontinuously supplied to the output when the power is switched from thesecond power source to the first power source.

In accordance with a preferred embodiment of the present invention,there is provided a method of switching power between a first powersource and a second power source. The method comprises providing powerto the output from the first power source with the MOSFET switch. Next,the switching time of the MOSFET switch is controlled by the RC network.Finally, the power is switched between the first power source and thesecond power source with the switch such that the second power sourcehas time to fully ramp-up before providing power to the output. Themethod further includes providing power from the first power source tothe output through the diode when the power is switched from the secondpower source to the first power source such that there is a continuousflow of power during the switch transition.

BRIEF DESCRIPTION OF THE DRAWINGS

These, as well as other features of the present invention, will becomemore apparent upon reference to the drawings wherein:

FIG. 1 is a circuit diagram of a prior art switching circuit capable ofproviding power between two sources; and

FIG. 2 is a circuit diagram of a switching circuit constructed inaccordance with the present invention and capable of providing powerbetween two sources.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings wherein the showings are for purposes ofillustrating a preferred embodiment of the present invention only, andnot for purposes of limiting the same, FIG. 2 is a circuit diagram of acircuit 100 capable of switching power between an internal power sourceand an external power source. The circuit 100 can switch between twoinput voltages V_(BAT) and V_(ADAPTER) to supply an output voltageV_(OUT). Typically, V_(BAT) is an internal power source supplied by abattery of the device and V_(ADAPTER) is an external power source.Referring to FIG. 2, the voltage V_(ADAPTER) is supplied by an externalDC adapter that includes an output stage diode D15, capacitor C12, andplug 202. Power is supplied from voltage source V_(DC) of the DC adapterto the plug 202 through diode D15. Typically, V_(DC) can supply amaximum of two amps. The plug 202 provides the DC voltage from the DCadapter at V_(ADAPTER).

The capacitor C12 of the DC adapter causes brownouts when the DC adapteris removed from the device. The voltage on C12 will not dischargeinstantly, thereby decreasing the voltage on the gate of U4 relativelyslowly. As such, during this period of time, the battery will not beable to supply enough current thereby resulting in a voltage drop atV_(OUT).

The power switching circuit 100 has a MOSFET switch U4 connected betweenvoltage source V_(BAT) and V_(ADAPTER). As previously mentioned, theMOSFET switch U4 has switching transistors 12 and 14, as well as diodes16 and 18. A FET switch is used to minimize the voltage drop from themain current path. The switch U4 has a voltage drop of about 50 mV, asopposed to a Shottky diode that has a drop of about 300 mV. Theswitching circuit 100 further includes resistor R102 and diode D10.

In order to provide brownout protection during switching from theinternal power source to the external power source, the circuit 100 hasan RC network of resistor R106 and capacitor C22. The RC network slowsdown the voltage rise on the gates of U4 when the DC adapter isconnected to the device. Accordingly, the voltage at V_(ADAPTER) canramp up before the voltage is present at V_(OUT), thereby preventingbrownouts during switching.

The values of R106 and C22 are chosen depending on the switchingconstant of the switch U4. Typically, R106 and C22 are chosen to give aRC time constant which is five times greater than the switching constantof the switch U4. This allows the voltage from the DC adaptor(V_(ADAPTER)) to fully ramp up by slowing down the voltage rise on thegate of U4.

The switching circuit 100 further includes diode D6 to prevent brownoutswhen the DC adaptor is removed from the device. The diode D6 will supplyenough current from V_(BAT) for proper operation while the switch is notfully on. The voltage drop from V_(BAT) to V_(OUT) is reduced by thevoltage drop across diode D6. By using a Shottky diode, it is possibleto minimize the voltage drop.

Additional modifications and improvements of the present invention mayalso be apparent to those of ordinary skill in the art. Thus, theparticular combination of parts described and illustrated herein isintended to represent only a certain embodiment of the presentinvention, and is not intended to serve as a limitation of alternativedevices within the spirit and scope of the invention.

1. A circuit for switching power from a first power source and a secondpower source to an output, the circuit comprising: a switching unit forcontrolling the flow of power from the first power source and the secondpower source to the output; and an RC network electrically connected tothe switching unit, the RC circuit being operative to delay theswitching of power from the first power source to the second powersource in order to provide a continuous supply of power.
 2. The circuitof claim 1 wherein the switching unit is a transistor and the RC networkdelays the switching of the transistor.
 3. The circuit of claim 2wherein the RC network is electrically connected to a gate of thetransistor.
 4. The circuit of claim 1 wherein the first power source isan internal power source and the second power source is an externalpower source.
 5. The circuit of claim 4 wherein the internal powersource is a battery and the second power source is a DC adapter.
 6. Thecircuit of claim 1 wherein a time constant for the RC network is greaterthan a switching constant for the switching unit.
 7. The circuit ofclaim 6 wherein the time constant for the RC network is about five timesgreater than the switching constant for the switching unit.
 8. A methodof switching power from a first power source and a second power sourceto an output with a circuit having a switching unit and a RC networkelectrically connected to the switching unit, the method comprising thesteps: (a) providing power to the output from the first power sourcewith the switching unit; (b) controlling the switching time of theswitching unit with the RC network; and (c) switching power to theoutput from the first power source to the second power source with theswitching unit such that the RC network delays switching to provide acontinuous flow of power from the first power source to the second powersource.
 9. The method of claim 8 wherein the first power source is aninternal power source and the second power source is an external powersource.
 10. The method of claim 9 wherein the first power source is abattery and the second power source is a DC adapter.
 11. The method ofclaim 8 wherein the switching unit is a transistor and step (b)comprises controlling the switching time of the transistor.
 12. Themethod of claim 11 wherein in step (b) the RC network controls thevoltage at the gate of the transistor in order to control the switchingtime.
 13. The method of claim 8 wherein a time constant for the RCnetwork is greater than a switching constant for the switching unit. 14.The method of claim 13 wherein the time constant for the RC network isabout five times greater than the switching constant for the switchingunit.
 15. A circuit for switching power between a first power source anda second power source to an output, the circuit comprising: switchingmeans for controlling the flow of power between the first power sourceand the second power source to the output; and switching delay meanselectrically connected to the switching means, the switching delay meansbeing operative to delay the switching of power between the first powersource and the second power source with the switching means such thatpower is continuous.
 16. The circuit of claim 15 wherein the switchingmeans is a MOSFET transistor and the switching delay means is a RCnetwork electrically connected to the MOSFET transistor.
 17. A circuitfor providing a continuous flow of power from a first and second powerSource to an output, the circuit comprising: a MOSFET switchelectrically connected to the first and second power sources andoperative to switch the output to one of the first and second powersources; and a RC network electrically connected to a gate of the MOSFETswitch, the RC network being operative to delay switching between thefirst and second power sources such that power can increase in thesecond power source before being connected to the output by the MOSFETswitch.
 18. The circuit of claim 17 wherein the first power source is aninternal battery and the second power source is an external DC adapter.19. The circuit of claim 17 wherein a time constant for the RC networkis greater than a switching constant for the MOSFET switch.
 20. Thecircuit of claim 19 wherein the time constant for the RC network is fivetimes greater than the switching constant for the MOSFET switch.